It is well known in the art to embed porous preforms at selected locations in aluminum castings and to impregnate them with the casting metal to enhance the properties (e.g., strength, wear resistance, creep, stiffness, thermal expansion, etc.) of the casting at such locations. The porous preforms typically comprise ceramic particles/fibers/whiskers bonded together (e.g., sintered) to form a porous body having a desired shape and a porosity of about 50% to about 98% by volume. Typical ceramics used include SiC, Al.sub.2 O.sub.3, SiO.sub.2, Al.sub.2 O.sub.3 /SiO.sub.2 blends and carbon fiber, inter alia. Porous metal preforms may also be used where the melting point of the preform metal is higher than the matrix metal forming the casting and impregnating the preform. In making such castings, the preform is positioned in the appropriate location within a mold cavity and impregnated/infiltrated with the molten metal forced into the cavity under pressure. This is typically accomplished using either the well known "squeeze casting" or "die casting" methods wherein permanent metal molds are used and pressure is applied to the molten metal in the mold cavity near the end of the stroke of a piston in the shot sleeve used to deliver metal to the mold cavity. Supplemental pistons, rods or the like may extend into the mold cavity to apply local pressure to the metal therein during solidification.
The "lost-foam" process is well known in the art and essentially involves (1) forming a pattern from a fugitive material, which pattern mimics the shape of the casting to be made, (2) depositing a porous ceramic/refractory coating on the pattern, (3) embedding the coated pattern in a bed of loose sand so as to define a mold cavity within the sand bed corresponding to the shape of the pattern, and (4) pouring molten metal into the mold cavity so as to destroy (e.g., vaporize) the fugitive pattern and fill the mold cavity left thereby with the metal. The pattern is provided with an extension which defines a sprue and runner system in the loose sand for introducing the metal to the mold cavity. The sprue portion of the extension typically stands higher than the high point of the cavity in order to provide a metallostatic head of metal sufficient to cause the metal to readily advance into the mold cavity and completely displace the fugitive pattern therein.
A commonly used mold pattern comprises a foam made from expanded polystyrene (EPS) beads steam-bonded together in an appropriate mold, which pattern vaporizes and/or liquifies and escapes the mold cavity through the refractory coating into the interstitial voids between the loose sand surrounding the pattern during casting. A metallostatic head of at least about 1 psi (i.e., about 10 inches high) above the high point of the mold cavity is typical for pattern made from EPS. Other fugitive materials useful as patterns for this process include polymethylmethacrylate (PMMA) and polyalkylene carbonate. Typically, porous protective refractory coatings on the pattern comprise silica, mica, and clay binders and serve to improve pattern stiffness, prevent sand erosion, improve casting surface finish, and aid in release of gas and liquid products from foam pyrolysis. The coatings may be applied by spraying or dipping.
Metal impregnated porous preform-containing castings have not heretofore been made using the "lost-foam" process. Accordingly, it is an object of the present invention to provide an improved "lost foam" process specifically adapted to forming castings having porous preforms embedded therein at selective locations thereof and filled with the metal forming the casting. This and other objects and advantages of the present invention will become more readily apparent from the following description thereof.